A bicycle frame with a hidden battery pack has a tube member and a battery pack. The tube member is hollow and has a peripheral surface, a receiving space, and a replacement opening. The receiving space is formed inside the tube member. The replacement opening is defined in the peripheral surface of the tube member and communicates with the receiving space. The battery pack is mounted in the receiving space and has multiple batteries. Each two adjacent batteries of the multiple batteries are connected to each other and are able to rotate relative to each other. Each battery of the multiple batteries is capable of being rotated or swung to pass through the replacement opening.
